The present invention relates to digital radiography and relates more specifically to a system wherein a radiation image is temporarily stored in a photostimulable phosphor screen.
Radiation image recording systems wherein a radiation image is recorded on a photostimulable phosphor screen by exposing said screen to image-wise modulated penetrating radiation are widely used nowadays.
The recorded image is reproduced by stimulating the exposed photostimulable phosphor screen by means of stimulating radiation and by detecting the light that is emitted by the phosphor screen upon stimulation and converting the detected light into an electrical signal representation of the radiation image.
Screens which are suitable for this application comprise e.g. a BaFX: Eu2+ phosphor or a divalent europium activated cesium halide phosphor wherein the halide is at least one of chloride and bromide or the like. The phosphor is deposited on top of a support layer such as a plastic film.
Some phosphors among which the above mentioned cesium halide phosphor are stable at normal temperature and humidity conditions but tend to be hygroscopic at high temperature and high humidity.
If a hygroscopic phosphor is exposed to moisture at elevated temperatures, the phosphor degrades. As a consequence sensitivity decreases. Since it is desirable that a radiographic system remains stable and has a long lifetime in all environmental conditions thus also in a hot and humid environment the above effects are unacceptable.
The most obvious way to avoid problems resulting from humidity is to coat the phosphor layer with a protective layer. However, it is common knowledge that such a protective layer leads to light piping. As a consequence of this effect the resolution of the screen degrades. This negative effect occurs even in case the protective layer is as thin as 10 micrometer.
It is an object of the present invention to provide a radiation detector wherein a radiation image is temporarily stored in a photostimulable phosphor screen that is stable and has a long life time under severe conditions regarding humidity and temperature.
Further objects of the present invention will become apparent from the description hereafter.
The above mentioned objects are realised by a radiation detector having the specific features defined in claim 1.
The inventors have found that high temperature on its own does not damage the system nor shorten its life cycle. Only the combination of high temperature together with high humidity degrades the screen.
The degradation is eliminated according to the present invention by providing a radiation detector comprising in addition to a phosphor screen the means for reading the radiation image from the screen and for occasionally erasing the screen. This detector is additionally provided with means for preventing humidity to enter the detector.
The phosphor screen is not taken out of the detector. It remains inside the enclosure of the detector during the exposure of the system to image-wise modulated radiation originating from irradiation of an object or a patient as well as during read out of the radiation image from an exposed screen and during occasional erasure of the screen between successive exposures.
According to one embodiment of the invention the detector is flushed by means of an an inert gas such as N2 gas, Ar gas, dried air or the like.
The gas may be circulated inside the system so that only a small quantity of the gas is needed.
Alternatively it can be pumped into the detector at one location and released at another location.
Specific features for preferred embodiments of the invention are set out in the dependent claims.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.